Virtually all states of insulin resistance are associated with compensatory hyperinsulinemia and often there is also Beta-cell hyperplasia. However, the factors which stimulate islet growth and the underlying glucose unresponsiveness, in these insulin resistant states remain unknown. Our lab has recently created several novel models of insulin resistance associated with varying degrees, and sometimes massive hyperplasia. Taking advantage of animals in which insulin receptors and insulin receptor substrates have been knocked out by homologous recombination, a model has been developed with heterozygote or homozygote defects in each of these pathways, as well as double homozygote and double heterozygote animals. The latter animals exhibit up to a 10-fold increase in Beta cell mass by the age of 6 months, despite the fact that they are completely euglycemic in both the fed and the fasted state. In addition, since global insulin receptor knockout result in a lethal phenotype, our laboratory has created mice with the pancreas tissue-specific disruption of the insulin receptor using the Cre-lox system, to study the role of the insulin receptor in the regulation of Beta-cell function. I will focus on a number of methods to study the growth and function of the islets in these models. Changes in islet growth will be characterized by bromodeoxyuridine labeling of the islets and using the mitogenic index. The secretory pattern of the islets will be characterized using common secretagogues. The insulin signaling pathway will be studied by western blot analysis using cell lines obtained from crossing the mice with the various knockout defects with mice expressing the SV40 large T antigen on a Beta-cell specific promoter. These experiments should provide an insightinto the role of the insulin receptor and its signaling pathway in Beta-cell growth and function.